In digital communications, many techniques heretofore have been employed for transmitting information from one place to another. In all of these techniques, a physical communications channel is used to connect the sending site with the receiving site. This channel may include a wire or cable, or it may involve a wireless, radio frequency atmospheric transmission path. In either case, the physical characteristics of the channel and of the equipment required for transmission and reception impose bandwidth limitations on the signals used for information transmission and on the transmission rates.
For example, two typical modulation techniques are pulse code modulation (PCM) and frequency-shift keying (FSK). With PCM, a digital word is encoded as a series of distinct pulses; it requires a bandwidth proportional to the sampling interval, since this determines a limit on the frequency content of the signal. With FSK, the modulating signal causes the instantaneous frequency of the output signal to switch between preselected values, without phase discontinuity; FSK requires a transmission bandwidth inversely proportional to the duration (i.e., epoch). When these bandwidth constraints are violated, the spectrum of the waveform at the output of the channel differs from the spectrum of the transmitted signal. Consequently, the information transmission rate (which depends on the sampling interval or tone duration) is limited by the available bandwidth. The bandwidth constraints may be violated, for example, by transmitting faster than the channel capacity will permit or by failing to adapt to changes in channel characteristics. With atmospheric channels, in particular, these constraints can become complicated to deal with since atmospheric transmissivity is a function of time as well as frequency.
When FSK and similar techniques are employed, one tone is sent at a time. Information may be sent bit-by-bit or in groupings of bits. The former possibility requires the use of only two possible tones for sending the data--a first tone for a "1" and a second tone for a "0" or, alternatively, the first tone to indicate that the bit values are unchanged and a second tone to indicate when there is a change of bit values (zero to one or one to zero). The second possibility, referred to as M-ary FSK, involves the use of M different signalling frequencies, one for each of M different message values. To decode a transmitted FSK signal, the receiver determines which of the possible signal frequencies is present in the received signal at each given moment; from the frequency content, it can regenerate the originating message element. Either way, the channel bandwidth places a lower bound on the required duration of the tonals.
One way to increase the amount of information which can be sent in a given time is to transmit more than one bit at a time, using frequency division multiplexing (FDM) techniques. According to FDM, each signal to be transmitted is assigned to a definite frequency or range of frequencies separated sufficiently from each other so that at the receiving station they may be distinguished as separate signals. FDM and FSK can be combined, with FSK being used to transmit each of the signals is the FDM system. Thus, N bits can be sent simultaneously, one in each of N frequency bands, or channels. With conventional techniques, this requires a channel whose bandwidth is at least as wide as the sum of the bandwidths of the N bands, often with some margin for channel spacing, as well.